Cycles Image as Plane: How to improve the quality of Image? Sampling rate?
Have you tried to raise the Map resolution in multiple importance under the world settings?



It can also be a UV map issue. The texture is stretched over the UV map's surfaces. If the surface is too small, it will become blurry and pixelated.
This also applies for the texture source itself, if it is to small it will become pixelated.

If you use light sources, you can make the shadows more sharp (become more crisp and clear) by reducing the light source's size (to something around 0.01).

With the shorthanded information you gave, I hope one of these suggestions will help you.

EDIT
Why are you importing it as emission? That will emit light and for sure as light spread, it will become blurry. Why not wrap the image on a plane as a texture?  

Color/contrast etc.. can be altered in the Compositing editor for the scene.